What causes pityriasis rosea?

Experts aren't sure what causes pityriasis rosea. Unlike many other skin conditions, it is not an allergic reaction or caused by a fungus or bacteria. It may be caused by a virus.

The rash does not appear to spread from person to person.

What are the symptoms?

Pityriasis rosea causes a rash.

The rash often begins with a single, round or oval, pink patch that is scaly with a raised border (herald patch). The size of the patch ranges from 2 cm (0.8 in.) to 10 cm (4 in.). The larger patches are more common. See a picture of a herald patch.

Days to weeks later, salmon-coloured, 1 cm (0.4 in.) to 2 cm (0.8 in.) oval patches appear in batches on the abdomen, chest, back, arms, and legs. Patches sometimes spread to the neck but rarely to the face.

Patches on the back are often vertical and angled to form a "Christmas tree" or "fir tree" appearance.

Mild itching is a problem for about half of the people who get the rash.

The rash usually lasts 6 to 8 weeks, but it can last up to several months.

In rare cases, the rash may take other forms. Rounded bumps (papular rash) may be seen in young children, pregnant women, and people who have dark skin. Blisters (vesicular rash) may be seen in infants and young children. In some people, the herald patch may not appear, or two herald patches may appear close together.

Before the herald patch appears, you may feel tired and as though you have a cold. You may have a headache, nausea, sore throat, and loss of appetite.

A rash similar to pityriasis rosea also can be caused by syphilis and by certain medicines such as antibiotics.

If you get a rash on the palms of your hands or the soles of your feet, see your doctor. This can be a sign of something more serious than pityriasis rosea.

How is pityriasis rosea diagnosed?

Your doctor will diagnose pityriasis rosea by looking at the rash. Diagnosis can be hard when only the herald patch is visible, because the condition is often mistaken for ringworm or eczema at this time. After the rash appears, diagnosis is generally clear.

If the diagnosis is unclear, your doctor may do a potassium hydroxide (KOH) test to make sure the rash is not caused by a fungal infection. A skin sample may be taken from the infected area and examined under the microscope (biopsy). If the diagnosis is unclear in a sexually active person, a test for syphilis is often done.

How is it treated?

Pityriasis rosea goes away without treatment. It usually lasts about 6 to 8 weeks. If the rash itches, you may wish to use skin lotions and lubricants to soothe itching. If symptoms are severe, your doctor may prescribe anti-inflammatory medicines such as corticosteroids to relieve itching and reduce the rash.

Although treatment isn't needed, antiviral medicines like acyclovir may shorten the time you have the rash, especially if you take them when the rash first starts.

Exposing the rash to sunlight may make it go away more quickly. But exposing your skin to the sun too long can result in sunburn and increase your risk of skin cancer.

If the rash lasts more than 3 months, contact your doctor.

To relieve itching at home:

Try to stay cool. Getting too warm and sweaty can make the rash and itching worse.

Avoid taking hot showers or baths. Keep the water as cool as you can tolerate.

Add a handful of oatmeal (ground to a powder) to your bath. Or you can try an oatmeal bath product, such as Aveeno.

Try an over-the-counter hydrocortisone cream for small itchy areas. Use the cream very sparingly on the face or genitals. Note: Do not use the cream on children younger than age 2 unless your doctor tells you to. Do not use in the rectal or vaginal area in children younger than age 12 unless your doctor tells you to.

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